1. Field of the Invention
This invention is concerned with an improved catalytic process for the demetalation and desulfurization of petroleum oils, preferably those residual fractions that have undesirably high metals and/or sulfur, and additionally decarbonization and/or reduction of CCR content. More particularly, the invention involves two catalysts with distinctly different pore sizes, arranged in a dual catalyst system that is especially effective for the demetalation, desulfurization, and decarbonization of petroleum oils. Both catalysts are exemplified by the cobalt-molybdenum-on-alumina type. The catalysts are extruded in selected quadrulobal shapes.
2. Description of the Prior Art
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals, sulfur, and/or CCR content. This comes about because practically all of the metals present in the original crude remain in the residual fraction, attached to polycyclic and highly aromatic compounds, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminants are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalytic processing such as catalytic cracking and hydrocracking. This is so because the metal contaminants deposit on the special catalysts for these processes and cause the formation of inordinate amounts of coke, dry gas, and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolitic operation known as coking. In this operation, the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of one to ten atmospheres. The economic value of the coke by-product is determined by its quality, especially its sulfur and metals content. Excessively high levels of these contaminants makes the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 p.p.m. (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high valued metallurgical, electrical, and mechanical applications.
Carbon residue may be determined by the Conradson Carbon Residue test. This test is important because Conradson carbon precursors generate surface coke on a catalyst, and the excess formation of coke upsets the heat balance of the catalytic cracking process. In general, higher-boiling range fractions contain more Conradson carbon or coke precursors. Light distillate oils may have a carbon residue less than 0.05 percent, but a vacuum residual oil may have a Conradson carbon value of 21 percent to 30 percent. Such a high Conradson carbon content, particularly when combined with excessive metals content and free radical content, essentially renders ineffective most conventional catalysts and catalytic treating processes.
The effect of such high carbon residue is that many residual petroleum feedstocks are unsuitable for use as FCC feedstocks, even if metals content and sulfur content are at acceptably low values.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content in many cases is unacceptable for ecological reasons.
At present, catalytic cracking is generally done utilizing hydrocarbon chargestocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since these cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree. to 1500.degree. F., at pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 1000 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor". This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25, or even 2.5 to 50, may be used to blend with or as all of the feedstock to a catalytic cracker, since chargestocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance with the newer fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions (having a metals factor of about 2.5 to 50) suitable for cracking chargestocks.
Metals and sulfur contaminants present similar problems with regard to hydrocracking operations which are typically carried out on chargestocks even lighter than those charged to a cracking unit. Hydrocracking catalyst is so sensitive to metals poisoning that a preliminary or first stage is often utilized for trace metals removal. Typical hydrocracking reactor conditions consist of a temperature of 400.degree. to 1000.degree. F. and a pressure of 100 to 3500 psig.
It is evident that there is considerable need for an efficient method to reduce the metals and/or sulfur content and/or residual carbon content of petroleum oils, and particularly of residual fractions of these oils. While the technology to accomplish this for distillate fractions has been advanced considerably, attempts to apply this technology to residual fractions generally fail due to very rapid deactivation of the catalyst, presumably by metals contaminants and coke deposition on the catalyst.
U.S. Pat. No. 3,730,879, issued May 1, 1973, discloses a two-bed catalytic process for the hydrodesulfurization of crude oil or a reduced fraction, in which at least 50 percent of the total pore volume of the first-bed catalytic consists of pores in the 100-200 Angstrom diameter range.
U.S. Pat. No. 3,830,720, issued Aug. 20, 1974, discloses a two-bed catalytic process for hydrocracking and hydrodesulfurizing residual oils, in which a small-pore catalyst is disposed upstream of a large-pore catalyst.
U.S. Pat. No. 3,876,523, issued Apr. 8, 1975, describes a novel catalyst and a process for catalytically demetalizing and desulfurizing hydrocarbon oils comprising residual fractions. This entire specification is incorporated herein by reference. The process described therein utilizes a catalyst comprising a hydrogenation component, such as cobalt and molybdenum oxides, composited on an alumina, at least a portion of which is in the delta and/or theta phase, with at least 60% of the pore volume of the catalyst in pores having a diameter of 100 Angstroms to 200 Angstroms, also having at least about 5% of the pore volume contributed by pores having a diameter greater than 500 Angstroms and having other characteristics as hereinafter described. As will be shown, although this catalyst is highly effective for demetalation of residual fractions and has good stability with time on stream, its utility is remarkably improved when this catalyst is employed in a particular manner in combination with a second catalyst having different critical characteristics. For convenience, a catalyst of the type described in U.S. Pat. No. 3,876,523 will be referred to herein as a first catalyst, it being understood that this first catalyst is to be situated upstream of the second catalyst having different characteristics.
A family of catalysts has been developed, based on a discovery, disclosed in U.S. Pat. No. 3,674,680, issued July 4, 1972, that metallic contaminants in a petroleum residuum that is hydroprocessed over small catalyst particles penetrate to a depth of 0.0085 inch from the particle surface and further based on the presumption that resid molecules do not penetrate much further. It was thus reasoned that an ideal catalyst would be one which has all points in the catalyst particle at a distance no greater than about 0.0085 inch from the particle surface, so as to eliminate any wasted catalyst material. It was therefore concluded that catalyst shape characteristics would desirably include concavity in addition to convexity, as in conventional spherical particles.
This family of catalysts is disclosed in British Pat. No. 1,446,175, issued Aug. 18, 1976, in Canadian Patent No. 1,007,187, issued Mar. 22, 1977, in Canadian Patent No. 1,015,350, issued Aug. 9, 1977, in U.S. Pat. No. 3,674,680, issued July 4, 1972, in U.S. Pat. No. 3,957,627, issued May 18, 1976, in U.S. Pat. No. 3,966,644, issued June 29, 1976, in U.S. Pat. No. 3,900,964, issued Nov. 9, 1976, in U.S. Pat. No. 4,028,227, issued June 7, 1977, in U.S. Pat. No. 4,118,310, issued Oct. 3, 1978, in U.S. Pat. No. 4,133,777, issued June 9, 1979, and in U.S. Pat. No. 4,153,539, issued May 8, 1979.
U.S. Pat. No. 4,153,539 discloses that improved hydrogen utilization and/or higher conversions of desired product is obtained in hydrotreating or hydrocracking processes when using amphora particles for hydrotreating of light hydrocarbon fractions, catalytic reforming, fixed-bed alkylation processes, and the like.
U.S. Pat. No. 4,133,777 teaches a process in which feed oil initially flows downwardly in trickle flow through a fixed bed of non-promoted catalysts, which removes a significant amount of feed metals and sulfur from the oil, and then passes downwardly through a fixed bed of promoted catalysts containing selected GROUP VI and GROUP VIII metals, with very little hydrocracking occurring in this combination process.
In the hydrodesulfurization process of U.S. Pat. No. 4,118,310, the temperature should be sufficiently low so that not more than 30% and preferably as little as 10% of the 650.degree. F.+ (343.degree. C.+) feed oil will be cracked to material boiling below 650.degree. F. (343.degree. C.), suitably using a hydrodesulfurization catalyst with four projections formed by four grooves and having a 1/36 inch (0.07 cm) diameter dimension.
In the hydrotreating process of U.S. Pat. Nos. 4,028,227 and 3,966,644 with polylobal catalyst particles, hydrodesulfurization is primarily accomplished with hydrocracking, nitrogen removal, and aromatic saturation occurring to a limited extent.
Using a cloverleaf-shaped catalyst for hydroprocessing an atmospheric resid, it is disclosed in U.S. Pat. No. 3,674,680 that the resid feed is cracked into lower boiling hydrocarbons and/or desulfurized and demetalized, with conversion of feed boiling above 650.degree. F. to products boiling below 650.degree. F. being typically 27 volume percent.
It was reported in U.S. Pat. No. 4,016,067, the entire specification of which is incorporated herein by reference, that hydrocarbon oils, preferably residual fractions, are catalytically hydroprocessed to very effectively remove both metals and sulfur and with particularly slow aging of the catalyst system by contacting the oil sequentially with two catalysts of different characteristics. The first catalyst, located upstream of the second catalyst, is characterized by having at least 60% of its pore volume in pores greater than 100 A. in diameter and other characteristics hereinafter specified. The second catalyst, located downstream with respect to the first catalyst, is characterized by having a major fraction of its pore volume in pores less than 100 A. in diameter.
The dual catalyst apparatus of U.S. Pat. No. 4,016,067 may be used to demetalize and/or desulfurize any hydrocarbon oil that has metals and/or sulfur content-undesirably high for a particular application. The dual catalyst apparatus is particularly effective for preparing low metals and/or low sulfur content feedstocks for catalytic cracking or for coking. In the processing to remove metals and sulfur, and hydrocarbon oil also is concomitantly enriched in hydrogen, making it an even more suitable chargestock for either of these processes.